1. Field of the Invention
The present invention relates generally to videotape recorders and more specifically to a high stability chrominance subcarrier regeneration network employing a reduced frequency voltage controlled oscillator circuit, the output of which is translated to the subcarrier frequency by a higher frequency crystal oscillator.
2. Description of the Prior Art
Color television systems such as those which conform to the NTSC (National Television System Committee) standard used in the United States process luminence (black and white) and chrominance (color) information separately. The chrominance information, which takes the form of a pair of difference signals commonly referred to as I and Q, is used to modulate a pair of 180.degree. displaced signals developed from a chrominance subcarrier signal. The pair of 180.degree. displaced signals are used to develop suppressed carrier modulated chrominance signals that are combined with the luminence information within the luminence band width to provide a means of demodulating the modulated signals. Also, a burst is included following each horizontal synchronization pulse. From the subcarrier burst signal the chrominance subcarrier signal can be regenerated to provide a reference for use in demodulating the suppressed carrier modulated chrominance signals and thus recover the chrominance information.
It is a relatively simple matter to regenerate the chrominance subcarrier signal from broadcast quality signals in that it is required that the subcarrier be maintained within .+-.10 Hertz of the center frequency of 3.579545 megahertz. Typically, the phase of each of the subcarrier burst signals is compared in a phase detector with the output of a voltage controlled crystal oscillator operating generally at the subcarrier frequency. The phase detector develops an error signal which is used to voltage control the frequency of the oscillator thereby locking it in frequency and phase to that of the subcarrier burst signals. When so phase locked, the voltage controlled crystal oscillator generates the required regenerated subcarrier signal at its output.
A more difficult regeneration problem is presented by video signals which have been tape recorded and reproduced. This is due to the degradation that results from large time perturbations that are introduced by the recordation/reproduction process which causes similar perturbations in the modulated chrominance signals.
A number of methods are known in the art for compensating for the time perturbations introduced by the recordation/reproduction process. One such method called the "color under" method employs circuitry which translates the frequency of the modulated chrominance signals to a substantially lower frequency such as 688 kilohertz before they are recorded. At this lower frequency, the signals are less susceptible to time base error degradation. The signals are then retranslated back to approximately 3.58 megahertz after they are reproduced.
In some prior art type recorder/reproducers, the modulated chrominance signals are further stabilized during this later retranslation process. Typically, these recorder/reproducers employ a mixer driven by a 4.27 megahertz correcting signal for correcting while retranslating the reproduced chrominance signals from 688 kilohertz to 3.58 megahertz. The correcting signal is derived from the retranslated reproduced chrominance signals by a gate which extracts the subcarrier burst signals therefrom, a phase comparator which compares the phase of a subcarrier burst with that of a 3.58 megahertz signal generated by a crystal oscillator to develop an error signal which is used to drive a 688 kilohertz voltage control oscillator, and another mixer for combining the output of the 688 kilohertz oscillator with that of the 3.58 megahertz crystal oscillator to develop the correcting signal.
It is important to note that the above-described circuit does not generate a regenerated subcarrier signal. The closest parallel thereto is a 4.27 megahertz correcting signal which is used both to correct and translate the reproduced chrominance signals.
A number of prior art methods exist for stabilizing reproduced modulated chrominance signals which have been directly recorded and reproduced without frequency translation. One method called the demodulation-remodulation method employs circuitry for developing a regenerated subcarrier signal from the reproduced modulated chrominance signal. As the name implies, the regenerated subcarrier is used to demodulate the reproduced modulated chrominance signals which are remodulated using a stable chrominance subcarrier signal.
Another method called the time base correcting method again relies on circuitry for developing a regenerated subcarrier signal. In general, the regenerated subcarrier signal and a horizontal synchronization signal also developed from the reproduced signals are used to clock the reproduced signals into a semiconductor memory from which they are extracted at a more constant rate in response to a stable reference signal.
Finally, a heterdyne method may be used to stabilize the reproduced modulated chrominance signals. Typically, two product signals are generated from a 3.58 megahertz reference signal. One of the product signals at 3.5 times the reference frequency is combined in a mixer with a regenerated subcarrier signal to produce a first sum signal. The other product signal at 4.5 times the reference frequency is combined in a mixer with the reproduced modulated chrominance signals to develop other sum signals. Finally, another mixer is employed to combine the sum signals and to develop therefrom difference signals which are the stabilized reproduced modulated chrominance signals.
It should be noted that the above-described stabilizing methods require the use of a regenerated subcarrier signal which is intimately locked in frequency and phase to the reference burst of the degradated composite video signal. Unfortunately, a phase lock loop employing a crystal controlled voltage controlled oscillator will not suffice in that it has inadequate capture band width. In other words, without extreme measures, the frequency of a 3.58 megahertz crystal oscillator cannot be pushed or pulled the amount necessary to follow the perturbations introduced by the recordation/reproduction process.
One prior art solution employs a regular free running voltage controlled modulator (not crystal controlled). Although effective, unless extremely stable, the oscillator will not properly lock to the subcarrier burst signals of the reproduced signal. The oscillator will instead lock to the side bands thereof. This is because when the original subcarrier oscillator signal is gated to develop each of the subcarrier bursts following each horizontal synchronization pulse, the subcarrier oscillator signal is modulated by the horizontal frequency generating many side bands near the subcarrier frequency of approximately 3.58 megahertz but differing therefrom and from each other by the horizontal frequency of approximately 15.75 kilohertz. Thus, should the voltage controlled oscillator through aging, shock, or temperature effects, drift in excess of approximately 1/4%, the subcarrier regeneration circuit may lock to one of the side bands. Heretofore, to prevent such an improper lock, it has been necessary to manually examine the resultant picture and to adjust the free running frequency of the voltage controlled oscillator until a proper lock is obtained, to employ a high degree of precise temperature compensation, or to resort to complex compound phase locked loops to insure proper lock, or similar means.